Lignin Carbonyl group

Formation and Elimination of Multiple Bond Functionalities. Reactions that involve the formation and elimination of multiple bond functional groups may significantly effect the color of residual lignin in bleached and unbleached pulps. The ethylenic and carbonyl groups conjugated with phenoHc or quinoid stmctures are possible components of chromophore or leucochromophore systems that contribute to the color of lignin. 

Later, Patterson and Hibbert (S3) stated that ultraviolet absorption of lignin corresponded with the absorptions of hydroxy derivatives of 4-(4,hydroxy-3,methoxyphenyl)-l-propanone and l-(4,hydroxy-3, 5,dimethoxyphenyl)-4-propanone. These authors, therefore, assumed the presence in lignin of a carbonyl group, or of an ethylenic double bond conjugated with an aromatic nucleus. They ascribed the 280 mp band to m-position freedom in the benzene ring and the 310 mp baud to a masked carbonyl group in conjugation with a benzene nucleus. 

The band at 280 mp persists in spite of such alterations in the molecule as are caused by inethylation, acetylation and treatment with sodium hydroxide. However, the absorption of the phenylhydrazones of spruce (33) and white Scots pine native lignins 111) are peculiar in that a second maximum occurs at 352 mp. Glading (33) suggests that the carbonyl group in spruce native lignin is highly enolized, and, therefore,... 

Since lignin is not a uniform entity, chemical criteria for its characterization have centred around analytical detenninations of its functional groups, e.g. total hydroxyl content, phenolic hydroxyls (56), methoxyl and other ether groups, benzyl alcohol groups (7a), carbonyl groups (6), etc., and estimations of its content of special structural features, e.g. phenylcoumaran units (5), biphenylyl linkages (123), etc. 

Figure 13. Contrasting effects upon their transformations by P. chrysosporium of introducing a-carbonyl groups into a dimeric nonphenolic 3-0-4 ether compound and the lignin macromolecule itself (72).

It should be noted that if the non-phenolic Ca-carbonyl compound contains an aromatic ring without Ca-carbonyl group (i.e., the structure should contain at least two aromatic rings), then the compound will be a substrate for lignin peroxidase and will be degraded according to the mechanisms discussed above. Degradation of this type of compounds can result, for example, in the formation of vanillin and vanillic acid derivatives (see compounds 7a and 8). As has been shown by Eriksson and coworkers... 

UV spectra of neutral solutions of ALCELL lignins exhibited maximum at 205-210 nm and at 275-281 nm which are characteristic of other lignin preparations. Alkali-neutral difference spectra exhibited three maxima at about 252-254 nm, 296-300 and 363-366 nm which indicate the presence of aromatic hydroxyl, a-conjugated hydroxyls, and conjugated carbonyl groups. The latter includes carbonyl groups in the a-position as well as those in cinnamaldehyde units mentioned above. The alkali-neutral difference spectrum of ALCELL lignins reduced with sodium borohydride shows an almost complete elimination of the peak at 360-366 nm and an increase... 

Thus, we can assume that two types of stilbene structures contribute to the chromophoric systems of kraft lignin. If further double bonds or carbonyl groups are conjugated with the stilbene as in XIII, the resulting structure is likely to have absorption within the visible region of the spectrum. 

B). Lignin contains carbonyl groups in various positions of the side chain. Hydroxymethyl groups can be introduced into the neighboring positions by substituting their activated hydrogen atoms (Tollens reaction). 

The side chains of lignins contain carbonyl groups as determined by NH2OH, HC1 reagent (/), and volumetric KBH4 method 21, 28). The distribution of the CO groups among different locations of the side chain was determined by spectrophotometric reduction —Ac method 6). The... 

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